Compact and high-speed wide-angle zoom lens system

ABSTRACT

A wide angle zoom lens system having a high level of brightness includes a zooming subsystem comprising first and second movable lens units and a fixed third lens unit. The first lens unit has at least one aspherical lens surface. In the preferred embodiment, a diaphragm is disposed between the second and third lens units and is displaced independently of the second lens group when zooming.

BACKGROUND OF THE INVENTION

The present invention relates to a zoom lens system for a still camera.More particularly, the present invention relates to a zoom lens systemthat attains a zoom ratio of about 2 by covering a half view angleranging from about 32° at the wide angle end to about 17° at thesemi-telephoto end and which is very compact despite its capability forattaining a brightness (i.e., large effective aperture) at F_(NO) of1:2.8 or in the range of F_(NO) 1:2.8 to 1:3.5.

Zoom lens systems that attain a zoom ratio of about 2 by covering halfview angles ranging from wide angle to semi-telephoto and which featureaperature ratios in the range of 1:3.5 to 1:4.5 are classified either asa two-group type which consists of a negative first lens group and apositive second lens group as disclosed in Japanese Patent Application(OPI) Nos. 64811/1984 and 142515/1984 (the term OPI as used herein meansan unexamined published Japanese patent application) or as a third-groupfixed type which includes a fixed negative third lens group additionallydisposed behind the combination of the negative first lens group and thepositive second lens group as shown in Japanese OPI Nos. 97016/1983,111013/1983 and 240217/1986. Zoom lens systems that feature an apertureratio on the order of 1:2.8 while attaining a zoom ratio of about 2 areshown in Japanese OPI Nos. 83543/1986 and 9550/1980 and they all relateto the two-group type.

Problems with the prior art zoom lens system featuring F_(NO) of 1:3.5to 1:4.5 will now be described. Japanese OPI No. 64811/1984 shows thatthe overall length of a lens system can be reduced by providing anaspherical surface in a first lens group which is composed of two lenselements. However, the power of the first lens group is so small that ithas to be displaced by a large amount during zooming and the overalllength of the lens system is by no means short at the wide-angle end. Inaddition, the focusing lens has to be advanced by too large an amount tomake the closest focusing distance short enough.

The zoom lens system shown in Japanese OPI No. 142515/1984 has a largeroverall length than the system disclosed in Japanese OPI No. 64811/1984because of the absence of any aspherical lens surfaces. Generallyspeaking, a zoom lens system of the two-group type does not allow thesecond lens group having a diaphragm stop to be moved by a smalleramount than in a lens system of the third-group fixed type having thesame overall length (as will be discussed later in this specification).

The zoom lens system of the third-group fixed type which is a ratherrecent version is essentially the same as the zoom lens system of thetwo-group type except that it additionally includes a rear converter.The system shown in Japanese OPI No. 97016/1983 has the advantage ofexperiencing a small distortion. On the other hand, probably because ofits intended application as a lens system for medium-format (6×4.5 cm or6×7 cm) camera, this lens system has the disadvantage that it has alarge overall length and requires many lens elements (especially in thefirst and third lens groups). The system shown in Japanese OPI No.111013/1983 has the advantage of small size but, on the other hand, thenegative powers of the first and third lens groups are so great thatsubstantial variations will occur in spherical aberration andastigmatism during zooming. Furthermore, the excessive power of thefirst lens group causes the disadvantage that the second lens group hasto be displaced by a large amount in spite of the use of the third lensgroup.

A zoom lens system that employs a negative third lens group which is notfixed but movable toward the object side together with the second lensgroup is proposed in many prior applications such as Japanese OPI Nos.26754/1979 and 59157/1979. However, as compared with the third-groupfixed type, this system has the disadvantage that the second lens grouphaving a diaphragm stop has to be displaced by a large amount and thatthe overall length of the system is by no means small.

With a view to solving these problems, the assignee of the presentinvention filed Japanese OPI No. 240217/1986 and proposed a zoom lenssystem having a shorter overall length than any of the prior artproducts. However, there still remains room for improvement of thissystem because its large F number is not well-suited to an obviousrequirement for a bright and compact system

Problems with the prior art lens systems featuring F_(NO) on the orderof 1:2.8 will now be discussed.

The zoom lens system shown in Japanese OPI No. 83543/1986 is very simplefor an aperture ratio of 1:2.8, since it is composed of 7 elements in 7units. However, it suffers the disadvantage of large overall length. Thesystem proposed in Japanese OPI No. 9550/1980 is also unsatisfactory inthat it is capable of attaining a zoom ratio of less than 1.6 and thatits overall length is by no means small. Each of these systems is of thetwo-group type and no system has been available in the art that is ofthe third-group fixed type and which features an aperture ratio on theorder of 1:2.8. This is probably because, with a lens system as brightas one having an F_(NO) of 1:2.8, effective aberrational compensationcannot be attained if comatic flare is increased by using a third lensgroup having a magnification greater than unity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a zoomlens system that ensures good performance in spite of having an F-numberF_(NO) of 1:2.8, or on the order of 1:2.8 to 1:3.5. The presentinvention attains this object by using a zoom lens system of thethird-group fixed type which has conventionally been considered to beunsuitable for the purpose of achieving a high level of brightness andby exploiting the two notable features of this type of lense, i.e., thatthe overall lens length and the amount of movement of the first andsecond lens groups (in particular, the second lens group having adiaphragm stop) can be reduced.

The compact and high-speed wide-angle zoom lens system of the presentinvention comprises a sub-system responsible for zooming that iscomposed, in order from the object side, of a first lens unit having anegative focal length and a second lens unit having a positive focallength, and a fixed third lens unit (a relay lens sub-system) that issubsequent to the zooming sub-system and which extends the focal lengththereof. The first lens unit has a negative and positive lensarrangement, and the second lens unit has a positive, negative andpositive lens arrangement. This lens system is capable of changing thefocal length while maintaining a constant focal position by mechanicallymoving the first and second lens units. The improvement of this zoomlens system is such that the first lens unit has at least one asphericalsurface and is of a three-unit-three-element composition consisting, inorder from the object side, of a negative meniscus lens element having aconcave surface of large curvature on the image side, a negative lenselement, and a positive lens element having a convex surface of largecurvature on the object side, and that the second lens unit has afour-unit-four-element composition consisting, in order from the objectside, of two positive lens elements each having a conex surface of largecurvature on the object side, a negative lens element having a surfaceof concave curvature on the image side, and a positive lens elementhaving a convex surface of large curvature on the image side. A furtherimprovement on this lens system is that it satisfies the followingconditions: ##EQU1## where, f₁ is the focal length of the first lensunit;

f₂ is the focal length of the second lens unit;

f_(L) is the focal length of the overall system at the narrow-angle end;

m₃ is magnification by the third lens unit;

N_(1n) is the average of the refractive indices at the d-line of the twonegative lens elements in the first lens unit; and

N_(2p) is the average of the refractive indices at the d-line of the twopositive lens elements in the second lens unit on its object side.

In a preferred embodiment, the third lens unit is simply composed of asingle negative lens element or consists of a cemented negative lens ofa mere one-unit-two-element composition wherein a positive lens elementis cemented to a negative lens element, or is in the form of a negativelens unit having a two-unit-two-element composition consisting of apositive and a negative lens element.

In another preferred embodiment, a fixed diaphragm stop is disposedbetween the second and third lens units in such a way that saiddiaphragm stop is displaced independently of the second lens unit by asmaller amount than said second lens unit when zooming is effected fromthe wide-angle end to the narrow-angle end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 3, 5, 7, 9, 11, 13 and 15 are simplified cross-sectional viewsof the lens systems constructed according to Examples 1 to 8,respectively, of the present invention;

FIGS. 2(a), 4(a), 6(a), 8(a), 10(a), 12(a), 14(a) and 16(a) are graphsplotting the aberrational curves obtained with the lens systems ofExamples 1 to 8, respectively, at the wide-angle end;

FIGS. 2(b), 4(b), 6(b), 8(b), 10(b), 12(b), 14(b) and 16(b) are graphsplotting the aberrational curves obtained with the lens systems ofExamples 1 to 8, respectively, at the middle-angle end; and

FIGS. 2(c), 4(c), 6(c), 8(c), 10(c), 12(c), 14(c) and 16(c) are graphsplotting the aberrational curves obtained with the lens systems ofExamples 1 to 8, respectively, at the narrow-angle end.

DETAILED DESCRIPTION OF THE INVENTION

As for the composition of the first and second lens units, the zoom lenssystem of the present invention employs a seven-unit-seven-elementcomposition which is generally similar to the system featuring an F_(NO)of 1:2.8 which is proposed in Japanese Patent Application (OPI) No.83543/1976. However, the powers of these lens units in the system of thepresent invention are made to be sufficiently greater than in the priorart system to permit the first lens unit to be displaced by a smalleramount. Furthermore, the system of the present invention has a fixedthird lens unit with a negative refractive power disposed behind thesecond lens unit and this is effective in reducing the amount ofdisplacement of the second lens unit.

Prior art zoom lens systems having an aperture ratio on the order ofF_(NO) 1:2.8 have had the problem that increased comatic flare developsif the powers of the first and second lens units are increased and thatan even greater comatic aberration occurs if a negative third lens unitis disposed behind the second lens unit. In the present invention, anaspherical lens surface is provided in the first lens unit and this iseffective not only in reducing the comatic aberration that develops inthe combination of the first and second lens units but also in ensuringsatisfactory performance even if a fixed third lens unit is added to thetwo-unit composition.

If an aperture ratio is made as bright as an F_(NO) of about 1:2.8without using any aspherical lens surface in the first unit and if theoverall length of the lens system is made short as in the presentinvention, the curvature of field will exhibit a great variation, i.e.,undercompensated at the wide-angle end and overcompensated at thenarrow-angle end. According to the present invention, an asphericalsurface is provided in the first lens unit which by its nature allowsoff-axis rays of light to pass at the wide-angle end at a differentposition that when it is at the narrow-angle end; this contributes tothe accomplishment of the following three objectives: a brighter Fnumber, compactness, and small variation in field curvature.

The amount of displacement of the second lens unit, ΔX₂, can beexpressed by: ##EQU2## where f_(S) is the focal length of the overallsystem at the wide-angle end. In a zoom lens system of the two-unittype, m₃ =1, so if the power of each lens unit is increased, ΔX₂ wouldalso increase because f₁ has a tendency to decrease by a greater amountthan f₂. On the other hand, in a zoom lens system of the third-unitfixed type where a negative lens is disposed in the third unit, m₃ >1and ΔX₂ can be made small even if the powers of the first and secondlens units are made larger than in the zoom lens system of the two-unittype.

As evidenced from the above-noted formula for the amount of displacementof the second lens unit, ΔX₂ can be made smaller by decreasing f₂ /|f₁ |in condition (1). If the lower limit of this condition is not reached,|f₁ | increases (since the second lens unit is a master lens, and f₂will not change greatly with lens type), and a circumstance favorablefor decreasing ΔX₂ will result. However, ΔX₁ (the amount of displacementof the first lens unit) will increase and the corresponding increase inthe overall lens length at the wide-angle end and the diameter of thefront lens unit makes it difficult to attain a compact lens system. Ifthe upper limit of condition (1) is exceeded, |f₁ | will decrease as inthe system shown in Japanese OPI No. 111013/1983 and ΔX₂ will increasedespite the incorporation of a fixed third-lens unit. Furthermore, greataberration changes will occur during zooming.

Condition (2) sets forth the requirement for the power of the first lensunit. If condition (1) is the only condition to be satisfied, one simplyneeds to be sure that the power of the first lens unit is smaller.Condition (2) is introduced in order to decrease the amount ofdisplacement of the first lens unit and the diameter of the front lensunit by putting a constraint on the power of the first lens unit. Thepower of the first lens unit in a zoom lens system intended to featurean aperture ratio of F_(NO) of about 1:2.8 or on the order of1:2.8-1:3.5 has to be made somewhat smaller than in a system with F_(NO)on the order of 1:3.5-1:4.5. If the lower limit of condition (2) is notreached, the power of the first lens unit is increased to an extent thatthe upper limit of condition (1) is exceeded, causing great aberrationalvariations during zooming and focusing as already mentioned. If theupper limit of condition (2) is exceeded, a circumstance favorable foraberrational compensation will result but a compact lens system cannotbe attained.

Condition (3) sets forth the requirement that should be satisfied by themagnifying power of the third lens unit. If the lower limit of thiscondition is not reached, ΔX₂ will become greater than in the case of azoom lens system of the two-unit type. If the upper limit of condition(3) is exceeded, a circumstance very favorable for decreasing ΔX₂ willresult, but on the other hand, the third lens unit cannot be composed ofa single negative lens element or a cemented negative lens of aone-unit-two-element composition since the comatic flare that hasdeveloped in the combination of the first and second lens units will beintensified by an amount corresponding to the magnifying power of thethird lens unit.

Condition (4) sets forth a requirement of the negative lens elements inthe first lens unit. The more compact a given lens system is, thegreater is the chance of the Petzval sum of assuming a negative value ifcondition (4) is not met. A notable problem is that astigmatism thatoccurs in the sagittal direction at the wide-angle end isovercompensated and cannot be effectively compensated.

Condition (5) sets forth the requirement that should be met by thepositive lens elements in the second lens unit on this object side. Inorder to attain a brightness on the order of F_(NO) of about 1:2.8, thezoom lens system of the present invention has to employ an opticalmaterial having a higher refractive index thant he materials used in thesystems featuring F_(NO) in the range of 1:3.5-1:4.5. The more compact agiven lens system is, the greater the variations that occur inaspherical and comatic aberrations during zooming if condition (5) isnot met.

In a preferred embodiment of the present invention, a fixed diaphragmstop is disposed between the second and third lens units and is capableof being moved independently from the second lens unit. This iseffective in eliminating the comatic flare of off-axis light over therange including the middle-angle and narrow-angle ends.

A number of examples of lens systems according to the present inventionwill now be described.

Examples 1 to 8 are described hereunder in terms of data sheets wherein:F_(NO) signifies an F number, f designates a focal length, w designatesa half view angle, f_(B) designates a back focus, r designates theradius of curvature of an individual lens surface, d designates a lensthickness or aerial distance between lenses, N designates the refractiveindex at the d-line of an individual lens element, and designates theAbbe number of an individual lens element.

The asphericity function is expressed by the following formula:

    ______________________________________                                        Example 1:                                                                    F.sub.NO = 1:2.8˜ 3.2˜3.6                                         f = 36.00˜50.00˜68.00                                             ω = 32.0˜23.3˜17.4 f.sub.B = 37.73                          ______________________________________                                        Surface No.                                                                              r         d         n      ν                                    ______________________________________                                        1          69.093    1.80      1.79952                                                                              42.2                                    2          25.618    8.85                                                     3          2872.647  1.70      1.77250                                                                              49.6                                    4          128.742   0.10                                                     5          34.09I    4.52      1.80518                                                                              25.4                                    6          58.975    0.20      1.58921                                                                              41.1                                    7          58.975    41.00˜17.67˜1.80                             8          36.490    3.73      1.77250                                                                              49.6                                    9          -293.149  2.70                                                     10         25.382    3.66      1.77250                                                                              49.6                                    11         189.788   0.69                                                     12         -160.084  6.87      1.80518                                                                              25.4                                    13         17.720    6.02                                                     14         987.761   2.58      1.62588                                                                              35.7                                    15         -36.915   1.0˜8.34˜17.77                               16         -180.493  1.50      1.58913                                                                              61.0                                    17         -434.076                                                           ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0             f.sub.2 / |f.sub.1 | = 0.564            a.sub.4 = 0.10317 × 10.sup.-5                                                               |f.sub.1 | / f.sub.L =  1.035           a.sub.6 = -0.22632 × 10.sup.-8                                                              m.sub.3 = 1.075                                           a.sub.8 = 0.12723 × 10.sup.-11                                                              N.sub.1n = 1.786                                          a.sub.10 = 0.0      N.sub.2p = 1.773                                          ______________________________________                                         ##EQU3##

    ______________________________________                                         Example 2:                                                                   F.sub.NO = 1:2.8˜ 3.2˜3.6                                         f = 36.20˜50.00˜68.00                                             ω = 31.9˜23.3˜17.4 f.sub.B = 38.00                          ______________________________________                                        Surface No.                                                                              r         d         n      ν                                    ______________________________________                                        1          64.868    1.80      1.83481                                                                              42.7                                    2          25.510    8.67                                                     3          787.929   1.70      1.69680                                                                              55.5                                    4          110.318   0.10                                                     5          33.811    4.15      1.80518                                                                              25.4                                    6          57.830    0.20      1.58921                                                                              41.1                                    7          57.830    42.02˜18.40˜2.00                             8          38.661    3.54      1.77250                                                                              49.6                                    9          -366.963  2.70                                                     10         25.467    3.78      1.77250                                                                              49.6                                    11         278.523   0.70                                                     12         -151.105  7.00      1.80518                                                                              25.4                                    13         18.090    6.18                                                     14         776.102   2.71      1.62588                                                                              35.7                                    15         -35.587   1.50˜8.55˜17.75                              16         -397.800  1.50      1.51633                                                                              64.1                                    17         397.800                                                            ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0             f.sub.2 / |f.sub.1 | = 0.562            a.sub.4 = 0.71640 × 10.sup.-6                                                               |f.sub.1 | / f.sub.L = 1.041            a.sub.6 = -0.24221 × 10.sup.-8                                                              m.sub.3 = 1.100                                           a.sub.8 = 0.77670 × 10.sup.-12                                                              N.sub.1n = 1.766                                          a.sub.10 = 0.0      N.sub.2p = 1.773                                          ______________________________________                                    

    ______________________________________                                        Example 3:                                                                    F.sub.NO = 1:2.8                                                              f = 36.20˜50.00˜68.00                                             ω = 32.0˜23.4˜17.4 f.sub.B = 38.61                          ______________________________________                                        Surface No.                                                                              r         d         n      ν                                    ______________________________________                                        1          61.738    1.85      1.79952                                                                              42.2                                    2          24.613    9.53                                                     3          -400.662  1.75      1.77250                                                                              49.6                                    4          147.834   0.10                                                     5          39.374    4.48      1.80518                                                                              25.4                                    6          84.698    0.20      1.58921                                                                              41.1                                    7          84.698    41.00˜18.00˜2.03                             8          51.342    4.61      1.77250                                                                              49.6                                    9          -206.967  2.70                                                     10         27.516    6.03      1.77250                                                                              49.6                                    11         -359.810  0.77                                                     12         -101.819  7.00      1.80518                                                                              25.4                                    13         19.739    6.22                                                     14         -914.731  3.28      1.62588                                                                              35.7                                    15         -34.321   1.50˜9.18˜19.20                              16         -35.000   1.70      1.58913                                                                              61.2                                    17         -39.055                                                            ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0             f.sub.2 / |f.sub.1 | = 0.598            a.sub.4 = 0.22413 × 10.sup.-6                                                               |f.sub.1 | / f.sub.L  = 1.007           a.sub.6 = -0.3214 × 10.sup.-8                                                               m.sub.3 = 1.075                                           a.sub.8 = 0.75129 × 10.sup.-12                                                              N.sub.1n = 1.786                                          a.sub.10 = 0.0      N.sub.2p = 1.773                                          ______________________________________                                    

    ______________________________________                                        Example 4:                                                                    F.sub.NO = 1:2.8˜ 3.2˜3.6                                         f = 36.20˜50.00˜68.00                                             ω = 32.0˜23.3˜17.4 f.sub.B = 37.00                          ______________________________________                                        Surface No.                                                                              r         d         n      ν                                    ______________________________________                                        1          63.377    2.00      1.83400                                                                              37.2                                    2          25.871    8.18                                                     3          789.315   1.90      1.77250                                                                              49.6                                    4          67.202    0.10                                                     5          31.770    0.50      1.58921                                                                              41.1                                    6          36.450    4.69      1.80518                                                                              25.4                                    7          77.825    37.91˜16.71˜2.00                             8          31.031    4.03      1.77250                                                                              49.6                                    9          -420.942  2.70                                                     10         22.268    3.43      1.77250                                                                              49.6                                    11         68.048    1.04                                                     12         -328.390  5.00      1.80518                                                                              25.4                                    13         16.156    4.27                                                     14         134.754   2.49      1.58144                                                                              40.7                                    15         -42.066   1.10˜8.32˜17.74                              16         -49.530   1.92      1.67270                                                                              32.1                                    17         -34.444   1.50      1.69680                                                                              55.5                                    18         -52.939                                                            ______________________________________                                        Aspherical 5th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 / |f.sub.1 | = 0.553           a.sub.4 = -0.25031 × 10.sup.-5                                                               |f.sub. 1 | / f.sub.L = 1.015          a.sub.6 = -0.39697 × 10.sup.-10                                                              m.sub.3 = 1.056                                          a.sub.8 = 0.33988 × 10.sup.-12                                                               N.sub.1n = 1.803                                         a.sub.10 = -0.46099 × 10.sup.-14                                                             N.sub.2p = 1.773                                         ______________________________________                                    

    ______________________________________                                        Example 5:                                                                    F.sub.NO = 1:2.8˜ 3.2˜3.6                                         f = 36.20˜50.00˜68.00                                             ω = 31.8˜23.3˜17.4 f.sub.B = 37.00                          ______________________________________                                        Surface No.                                                                              r         d         n      ν                                    ______________________________________                                        1          63.008    2.00      1.83400                                                                              37.2                                    2          25.021    8.47                                                     3          657.128   0.20      1.58921                                                                              41.1                                    4          657.128   1.90      1.77250                                                                              49.6                                    5          82.683    0.10                                                     6          35.270    5.07      1.80518                                                                              25.4                                    7          80.294    40.56˜17.80˜2.00                             8          33.498    3.91      1.77250                                                                              49.6                                    9          -380.690  2.70                                                     10         25.346    3.4       1.77250                                                                              49.6                                    11         107.426   0.96                                                     12         -171.93   6.68      1.80518                                                                              25.4                                    13         17.562    4.49                                                     14         155.604   2.61      1.58144                                                                              40.7                                    15         -37.313   1.30˜8.32˜17.48                              16         -98.218   1.81      1.67270                                                                              32.1                                    17         -55.574   1.50      1.69680                                                                              55.5                                    18         -135.135                                                           ______________________________________                                        Aspherical 3rd Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 / |f.sub.1 | = 0.555           a.sub.4 = -0.10867 × 10.sup.-5                                                               |f.sub.1 |  / f.sub.L = 1.033          a.sub.6 = 0.38254 × 10.sup.-8                                                                m.sub.3 = 1.090                                          a.sub.8 = -0.75690 × 10.sup.-11                                                              N.sub.1n = 1.803                                         a.sub.10 = 0.11202 × 10.sup.-13                                                              N.sub.2p = 1.773                                         ______________________________________                                    

    ______________________________________                                        Example 6:                                                                    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             ω = 32.0˜23.3˜17.3 f.sub.B = 37.80                   Surface No.                                                                            r          d            n     ν                                   ______________________________________                                        1        89.074     2.00         1.79952                                                                             42.2                                   2        26.111     8.17                                                      3        569.990    1.90         l.77250                                                                             49.6                                   4        121.960    0.10                                                      5        35.687     4.73         1.80518                                                                             25.4                                   6        68.451     0.20         1.58921                                                                             41.1                                   7        68.451     41.88˜18.34˜2.00                              8        34.351     3.87         1.77250                                                                             49.6                                   9        -405.893   2.70                                                      10       26.060     3.47         1.77250                                                                             49.6                                   11       120.217    0.91                                                      12       -142.298   7.00         1.80518                                                                             25.4                                   13       18.320     4.78                                                      14       136.211    2.84         1.58144                                                                             40.7                                   15       -34.654    1.10˜7.83˜16.61                               16       207.252    1.92         1.67270                                                                             32.1                                   17       -224.694   0.0                                                       18       -224.694   1.50         1.69680                                                                             55.5                                   19       95.103                                                               ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.l |= 0.560             a.sub.4 = 0.81081 × 10.sup.-6                                                                |f.sub.1 |/f.sub.L = 1.017             a.sub.6 = -0.40136 × 10.sup.-8                                                               m.sub.3 = 1.150                                          a.sub.8 =  0.28045 × 10.sup.-11                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

    ______________________________________                                        Example 7:                                                                    ______________________________________                                               F.sub.NO = 1:2.8                                                              f = 36.20˜50.00˜68.00                                             ω= 32.1˜23.4˜17.5 f.sub.B = 37.80                    Surface No.                                                                            r          d            n     ν                                   ______________________________________                                        1        69.146     2.00         1.79952                                                                             42.2                                   2        25.460     8.87                                                      3        7165.208   1.90         1.77250                                                                             49.6                                   4        129.006    0.35                                                      5        36.743     4.33         1.80518                                                                             25.4                                   6        66.512     0.20         1.58921                                                                             41.1                                   7        66.512     39.69˜17.45˜2.00                              8        38.773     4.92         1.77250                                                                             49.6                                   9        -364.554   2.70                                                      10       25.742     4.91         1.77250                                                                             49.6                                   11       178.517    1.19                                                      12       -154.655   5.30         1.80518                                                                             25.4                                   13       18.497     5.83                                                      14       215.167    3.37         1.58144                                                                             40.7                                   15       -36.431    1.10˜8.73˜18.67                               16       -71.605    1.66         1.60342                                                                             38.0                                   17       -49.094    0.0                                                       18       -49.094    1.50         1.69680                                                                             55.5                                   19       -79.102                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /| f.sub.1 |= 0.591            a.sub.4 = 0.51170 × 10.sup.-6                                                                |f.sub.1 | /f.sub.L = 0.999            a.sub.6 = -0.26068 × 10.sup.-8                                                               m.sub.3 = 1.070                                          a.sub.8 =  0.56946 × 10.sup.-12                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

    ______________________________________                                        Example 8:                                                                    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             ω= 32.0˜23.4 ˜17.4 f.sub.B = 37.80                   Surface No.                                                                            r          d            n     ν                                   ______________________________________                                        1        119.151    2.00         1.79952                                                                             42.2                                   2        25.366     7.14                                                      3        129.090    1.90         1.77250                                                                             49.6                                   4        92.893     0.10                                                      5        38.489     4.65         1.80518                                                                             25.4                                   6        83.341     0.20         1.58921                                                                             41.1                                   7        83.341     40.44˜17.75˜2.00                              8        33.808     3.93         1.77250                                                                             49.6                                   9        -401.330   2.70                                                      10       26.422     3.46         1.77250                                                                             49.6                                   11       125.719    0.87                                                      12       -150.833   7.00         1.80518                                                                             25.4                                   13       18.279     4.11                                                      14       309.005    2.59         1.58144                                                                             40.7                                   15       -35.631    1.10˜8.54˜18.24                               16       -131.276   1.81         1.71736                                                                             29.5                                   17       -69.939    0.50                                                      18       -48.221    1.50         1.69680                                                                             55.5                                   19       -81.622                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.566            a.sub.4 = -0.15414 × 10.sup.-5                                                               |f.sub.1 | /f.sub.L = 1.041            a.sub.6 = -0.39349 × 10.sup.-8                                                               m.sub.3 =  1.050                                         a.sub.8 = -0.18128 × 10.sup.-11                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

What is claimed is:
 1. A compact and high-speed wide-angle zoom lenssystem having a sub-system responsible for zooming and an F-number of1:2.8 to 1:3.5, said sub-system comprising, in order from the objectside: a first lens unit having a negative focal length, a second lensunit having a positive focal length and a fixed third lens unit forminga relay lens sub-system that is subsequent to the zooming sub-system andwhich extends the focal length thereof, said lens system being capableof changing its focal length while maintaining a constant focal positionby mechanically moving the first and second lens units, the improvementwherein the first lens unit has at least one aspherical surface and isof a three-unit-three-element composition comprising, in order from theobject side, a negative meniscus lens element having a concave surfaceof large curvature on the image side, a negative lens element, and apositive lens element having a convex surface of large curvature on theobject side, and the second lens unit has a four-unit-four-elementcomposition comprising, in order from the object side, two positive lenselements, each having a convex surface of large curvature on the objectside, a negative lens element having a surface of concave curvature onthe image side, and a positive lens element having a convex surface oflarge curvature on the image side, said lens system further satisfyingthe following conditions: (1) 0.5<f₂ /f₁ <0.63(2) 0.9<f₁ /f_(L) <1.2 (3)1.0<m₃ <1.2 (4) 1.7<N_(1n) (5) 1.7<N_(2p) where, f₁ is the focal lengthof the first lens unit; f₂ is the focal length of the second lens unit;F_(L) is the focal length of the overall system at the narrow-angle end;m₃ is magnification by the third lens unit; N_(1n) is the average of therefractive indices at the d-line of the two negative lens elements inthe first lens unit; and N_(2p) is the average of the refractive indicesat the d-line of the two positive lens elements in the second lens uniton its object side.
 2. A zoom lens system according to claim 1 whereinthe third lens unit consists of a single negative lens element.
 3. Azoom lens system according to claim 1 wherein the third lens unitconsists of a cemented negative lens of a one-unit-two-elementcomposition wherein a positive lens element is cemented to a negativelens element.
 4. A zoom lens system according to claim 1 wherein thethird lens unit is in the form of a negative lens unit having atwo-unit-two-element composition consisting of a positive and a negativelens element.
 5. A zoom lens system according to any one of claims 1 to4 wherein a diaphragm stop is disposed between the second and third lensunits in such a way that said diaphragm stop is displaced independentlyof the second lens unit by a smaller amount than said second lens unitwhen zooming is effected from the wide-angle end to the narrow-angleend.
 6. A zoom lens system according to claim 1, further satisfying thefollowing chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.00˜50.00˜68.00                                             w = 32.0˜23.3˜17.4 f.sub.B = 37.73                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        69.093     1.80         1.79952                                                                             42.2                                   2        25.618     8.85                                                      3        2872.647   1.70         1.77250                                                                             49.6                                   4        128.742    0.10                                                      5        34.091     4.52         1.80518                                                                             25.4                                   6        58.975     0.20         1.58921                                                                             41.1                                   7        58.975     41.00˜17.67˜1.80                              8        36.490     3.73         1.77250                                                                             49.6                                   9        -293.149   2.70                                                      10       25.382     3.66         1.77250                                                                             49.6                                   11       189.788    0.69                                                      12       -160.084   6.87         1.80518                                                                             25.4                                   13       17.720     6.02                                                      14       987.761    2.58         1.62588                                                                             35.7                                   15       -36.915    1.0˜8.34˜17.77                                16       -180.493   1.50         1.58913                                                                             61.0                                   17       -434.076                                                             ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.564            a.sub.4 = 0.10317 × 10.sup.-5                                                                |f.sub.1 | /f.sub.L = 1.035            a.sub.6 = -0.22632 × 10.sup.-8                                                               m.sub.3 = 1.075                                          a.sub.8 = 0.12723 × 10.sup.-11                                                               N.sub.1n  = 1.786                                        a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 7. A zoom lens system according toclaim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             w = 31.9˜23.3˜17.4 f.sub.B = 38.00                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        64.868     1.80         1.83481                                                                             42.7                                   2        25.510     8.67                                                      3        787.929    1.70         1.69680                                                                             55.5                                   4        110.318    0.10                                                      5        33.811     4.15         1.80518                                                                             25.4                                   6        57.830     0.20         1.58921                                                                             41.1                                   7        57.830     42.02˜18.40˜2.00                              8        38.661     3.54         1.77250                                                                             49.6                                   9        -366.963   2.70                                                      10       25.467     3.78         1.77250                                                                             49.6                                   11       278.523    0.70                                                      12       -151.105   7.00         1.80518                                                                             25.4                                   13       18.090     6.18                                                      14       776.102    2.71         1.62588                                                                             35.7                                   15       -35.587    1.50˜8.55˜17.75                               16       ˜397.800                                                                           1.50         1.51633                                                                             64.1                                   17       397.800                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.562            a.sub.4 = 0.71640 × 10.sup.-6                                                                |f.sub.1 | /f.sub.L = 1.041            a.sub.6 = -0.24221 × 10.sup.-8                                                               m.sub.3 = 1.100                                          a.sub.8 = 0.77670 × 10.sup.-12                                                               N.sub.1n =  1.766                                        a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 8. A zoom lens system according toclaim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8                                                              f = 36.20˜50.00˜68.00                                             w = 32.0˜23.4˜17.4 f.sub.B = 38.61                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        61.738     1.85         1.79952                                                                             42.2                                   2        24.613     9.53                                                      3        -400.662   1.75         1.77250                                                                             49.6                                   4        147.834    0.10                                                      5        39.374     4.48         1.80518                                                                             25.4                                   6        84.698     0.20         1.58921                                                                             41.1                                   7        84.698     41.00˜18.00˜2.03                              8        51.342     4.61         1.77250                                                                             49.6                                   9        -206.967   2.70                                                      10       27.516     6.03         1.77250                                                                             49.6                                   11       -359.810   0.77                                                      12       -101.819   7.00         1.80518                                                                             25.4                                   13       19.739     6.22                                                      14       -914.731   3.28         1.62588                                                                             35.7                                   15       -34.321    1.50˜9.18˜19.20                               16       -35.000    1.70         1.58913                                                                             61.2                                   17       -39.055                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 1|f.sub.1 | = 0.598            a.sub.4 = 0.22413 × 10.sup.-6                                                                |f.sub.1 | /f.sub.L = 1.007            a.sub.6 = -0.32184 × 10.sup.-8                                                               m.sub.3 = 1.075                                          a.sub.8 = 0.75129 × 10.sup.-12                                                               N.sub.1n  = 1.786                                        a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 9. A zoom lens system according toclaim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             w = 32.0˜23.3˜17.4 f.sub.B = 37.00                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        63.377     2.00         1.83400                                                                             37.2                                   2        25.871     8.18                                                      3        789.315    1.90         1.77250                                                                             49.6                                   4        67.202     0.10                                                      5        31.770     0.50         1.58921                                                                             41.1                                   6        36.450     4.69         1.80518                                                                             25.4                                   7        77.825     37.91˜16.71˜2.00                              8        31.031     4.03         1.77250                                                                             49.6                                   9        -420.942   2.70                                                      10       22.268     3.43         1.77250                                                                             49.6                                   11       68.048     1.04                                                      12       -328.390   5.00         1.80518                                                                             25.4                                   13       16.156     4.27                                                      14       134.754    2.49         1.58144                                                                             40.7                                   15       -42.066    1.10˜8.32˜17.74                               16       -49.530    1.92         1.67270                                                                             32.1                                   17       -34.444    1.50         1.69680                                                                             55.5                                   18       -52.939                                                              ______________________________________                                        Aspherical 5th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.553            a.sub.4 = -0.25031 × 10.sup.-5                                                               |f.sub.1 |/f.sub.L = 1.015             a.sub.6 = -0.39697 × 10.sup.-10                                                              m.sub.3 = 1.056                                          a.sub.8 = 0.33988 × 10.sup.-12                                                               N.sub. 1n = 1.803                                        a.sub.10 = -0.46099 ×10.sup.-14                                                              N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 10. A zoom lens system accordingto claim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             w = 31.8˜23.3˜17.4 f.sub.B = 37.00                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        63.008     2.00         1.83400                                                                             37.2                                   2        25.021     8.47                                                      3        657.128    0.20         1.58921                                                                             41.1                                   4        657.128    1.90         1.77250                                                                             49.6                                   5        82.683     0.10                                                      6        35.270     5.07         1.80518                                                                             25.4                                   7        80.294     40.56˜17.80˜2.00                              8        33.498     3.91         1.77250                                                                             49.6                                   9        -380.690   2.70                                                      10       25.346     3.44         1.77250                                                                             49.6                                   11       107.426    0.96                                                      12       -171.983   6.86         1.80518                                                                             25.4                                   13       17.562     4.49                                                      14       155.604    2.61         1.58144                                                                             40.7                                   15       -37.313    1.30˜8.32˜17.48                               16       -98.218    1.81         1.67270                                                                             32.1                                   17       -55.574    1.50         1.69680                                                                             55.5                                   18       -135.135                                                             ______________________________________                                        Aspherical 3rd Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.555            a.sub.4 = -0.10867 × 10.sup.-5                                                               |f.sub.1 |/f.sub.L = 1.033             a.sub.6 = 0.38254 × 10.sup.-8                                                                m.sub.3 = 1.090                                          a.sub.8 = -0.75690 × 10.sup.-11                                                              N.sub. 1n = 1.803                                        a.sub.10 = 0.11202 × 10.sup.-13                                                              N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 11. A zoom lens system accordingto claim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             w = 32.0˜23.3˜17.3 f.sub.B = 37.80                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        89.074     2.00         1.79952                                                                             42.2                                   2        26.111     8.17                                                      3        569.990    1.90         1.77250                                                                             49.6                                   4        121.960    0.10                                                      5        35.687     4.73         1.80518                                                                             25.4                                   6        68.451     0.20         1.58921                                                                             41.1                                   7        68.451     41.88˜18.34˜2.00                              8        34.351     3.87         1.77250                                                                             49.6                                   9        -405.893   2.70                                                      10       26.060     3.47         1.77250                                                                             49.6                                   11       120.217    0.91                                                      12       -142.298   7.00         1.80518                                                                             25.4                                   13       18.320     4.78                                                      14       136.211    2.84         1.58144                                                                             40.7                                   15       -34.654    1.10˜7.83˜16.61                               16       207.252    1.92         1.67270                                                                             32.1                                   17       -224.694   0.0                                                       18       -224,694   1.50         1.69680                                                                             55.5                                   19       95.103                                                               ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.560            a.sub.4 = 0.81081 × 10.sup.-6                                                                |f.sub.1 |/f.sub.L = 1.017             a.sub.6 = -0.40136 × 10.sup.-8                                                               m.sub.3 = 1.150                                          a.sub.8 = 0.28045 × 10.sup.- 11                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 12. A zoom lens system accordingto claim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8                                                              f = 36.20˜50.00˜68.00                                             w = 32.1˜23.4˜17.5 f.sub.B = 37.80                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        69.146     2.00         1.79952                                                                             42.2                                   2        25.460     8.87                                                      3        7165.208   1.90         1.77250                                                                             49.6                                   4        129.006    0.35                                                      5        36.743     4.33         1.80518                                                                             25.4                                   6        66.512     0.20         1.58921                                                                             41.1                                   7        66.512     39.69˜17.45˜2.00                              8        38.773     4.92         1.77250                                                                             49.6                                   9        -364.554   2.70                                                      10       25.742     4.91         1.77250                                                                             49.6                                   11       178.517    1.19                                                      12       -154.655   5.30         1.80518                                                                             25.4                                   13       18.497     5.83                                                      14       215.167    3.37         1.58144                                                                             40.7                                   15       -36.431    1.10˜8.73˜18.67                               16       -71.605    1.66         1.60342                                                                             38.0                                   17       -49.094    0.0                                                       18       -49.094    1.50         1.69680                                                                             55.5                                   19       -79.102                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 | = 0.591            a.sub.4 = 0.51170 × 10.sup.-6                                                                |f.sub.1 |/f.sub.L = 1.999             a.sub.6 = -0.26068 ×10.sup.-8                                                                m.sub.3 = 1.070                                          a.sub.8 = 0.56946 × 10.sup.- 12                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.
 13. A zoom lens system accordingto claim 1, further satisfying the following chart:

    ______________________________________                                               F.sub.NO = 1:2.8˜3.2˜3.6                                          f = 36.20˜50.00˜68.00                                             w = 32.0˜23.4˜17.4 f.sub.B = 37.80                         Surface No.                                                                            r          d            n     v                                      ______________________________________                                        1        119.151    2.00         1.79952                                                                             42.2                                   2        25.366     7.14                                                      3        129.090    1.90         1.77250                                                                             49.6                                   4        92.893     0.10                                                      5        38.489     4.65         1.80518                                                                             25.4                                   6        83.341     0.20         1.58921                                                                             41.1                                   7        83.341     40.44˜17.75˜2.00                              8        33.808     3.93         1.77250                                                                             49.6                                   9        -401.330   2.70                                                      10       26.422     3.46         1.77250                                                                             49.6                                   11       125.719    0.87                                                      12       -150.833   7.00         1.80518                                                                             25.4                                   13       18.279     4.11                                                      14       309.005    2.59         1.58144                                                                             40.7                                   15       -35.631    1.10˜8.54˜18.24                               16       -131.276   1.81         1.71736                                                                             29.5                                   17       -69.939    0.50                                                      18       -48.221    1.50         1.69680                                                                             55.5                                   19       -81.622                                                              ______________________________________                                        Aspherical 7th Surface                                                        ______________________________________                                        K = 0.0              f.sub.2 /|f.sub.1 |= 0.566             a.sub.4 = -0.15414 × 10.sup.-5                                                               |f.sub.1 |/ f.sub.L = 1.041            a.sub.6 = -0.39349 × 10.sup.-8                                                               m.sub.3 = 1.050                                          a.sub.8 = -0.18128 × 10.sup.-11                                                              N.sub.1n = 1.786                                         a.sub.10 = 0.0       N.sub.2p = 1.773                                         ______________________________________                                    

where F_(NO) signifies an F number, f designates a focal length, wdesignates a half view angle, f_(B) designates a back focus, r is theradius of curvature of an individual lens surface, d designates a lensthickness or the aerial distance between lenses, N is the refractiveindex at the d-line of an individual lens element, and v is the Abbenumber of an individual lens element.